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Divergence of paralogous pairs, resulting from gene duplication, plays an important role in the evolution of specialized or novel gene functions. Analysis of selected duplicated pairs has elucidated some of the mechanisms underlying the functional diversification of  Saccharomyces cerevisiae  ( S .  cerevisiae ) paralogous genes. Similar studies of the orthologous pairs extant in pre-whole genome duplication yeast species, such as  Kluyveromyces lactis  ( K. lactis ) remain to be addressed. The genome of  K. lactis , an aerobic yeast, includes gene pairs generated by sporadic duplications. The genome of this organism comprises the  KlLEU4  and  KlLEU4BIS  paralogous pair, annotated as putative α-isopropylmalate synthases (α-IPMSs), considered to be the orthologs of the  S. cerevisiae ScLEU4 / ScLEU9  paralogous genes. The enzymes encoded by the latter two genes are mitochondrially located, differing in their sensitivity to leucine allosteric inhibition resulting in  Sc Leu4- Sc Leu4 and  Sc Leu4- Sc Leu9 sensitive dimers and  Sc Leu9- Sc Leu9 relatively resistant homodimers. Previous work has shown that, in a  Scleu4 Δ mutant,  ScLEU9  expression is increased and assembly of  Sc Leu9- Sc Leu9 leucine resistant homodimers results in loss of feedback regulation of leucine biosynthesis, leading to leucine accumulation and decreased growth rate. Here we report that: (i)  K. lactis  harbors a sporadic gene duplication, comprising the  KlLEU4 , syntenic with  S. cerevisiae ScLEU4  and  ScLEU9 , and the non-syntenic  KlLEU4BIS , arising from a pre-WGD event. (ii) That both,  KlLEU4  and  KlLEU4BIS  encode leucine sensitive α-IPMSs isozymes, located in the mitochondria ( Kl Leu4) and the cytosol ( Kl Leu4BIS), respectively. (iii) That both,  KlLEU4  or  KlLEU4BIS  complement the  Scleu4 Δ  Scleu9 Δ leucine auxotrophic phenotype and revert the enhanced  ScLEU9  transcription observed in a  Scleu4 Δ  ScLEU9  mutant. The  Scleu4 Δ  ScLEU9  growth mutant phenotype is only fully complemented when transformed with the syntenic  KlLEU4  mitochondrial isoform.  KlLEU4  and  KlLEU4BIS  underwent a different diversification pathways than that leading to  ScLEU4 / ScLEU9 .  KlLEU4  could be considered as the functional ortholog of  ScLEU4 , since its encoded isozyme can complement both the  Scleu4 Δ  Scleu9 Δ leucine auxotrophy and the  Scleu4 Δ  ScLEU9  complex phenotype.

frontiers in microbiology

In Kluyveromyces lactis a Pair of Paralogous Isozymes Catalyze the First Committed Step of Leucine Biosynthesis in Either the Mitochondria or the Cytosol